The ability of cells to respond to each other and the substratum (forms of cell social behavior) is an important part of normal growth and development. Failures in these responses are believed to be involved in metastasis and the loss of contact inhibition of growth, forms of cell social behavior displayed by tumor cells. The mechanism by which cell social behavior is regulated remains unknown. One common model for the regulation of cell social behavior is that it is mediated by cell surface carbohydrate, in the form of glycolipid and/or glycoprotein. I have proposed that heparan sulfate, a member of the third class of cell surface carbohydrates, the glycoaminoglycans, is involved in the expression of cell social behavior. This proposal is based, in part, upon my observation that there is a decrease in the sulfate content of cell surface HS as a result of transformation of mouse cells by either DNA (Papova-) or RNA (Leuko-) tumor viruses. Specifically, I have proposed that the cell surface HS forms a network with itself or other surface molecules, and that this network interacts with the cytoskeleton as well as with molecules on adjacent cells (intercellular communication). My current research is directed towards critical testing of these proposals through analyses of the interactions of cell surface heparan sulfate with membrane molecules as well as toward correlation of heparan sulfate structures with specific forms of cell social behavior.